The present invention relates to a laser which is excited by a semiconductor element such as an LD (laser diode) or LED (light-emitting diode), and more particularly to an improvement in the structure for its excitation.
FIG. 1 shows the structure of the prior art LD excited solid state laser, which is disclosed, for example, in U.S. Pat. No. 3,624,545. In the figure, reference numeral 1 denotes an LD; 2 is a laser medium, for example, a YAG rod; 3 and 4 are a totally reflecting film and a partially reflecting film, respectively, formed on end faces of the laser medium 2; and 5 is a reflecting mirror.
An excitation light beam emitted by the LD 1 is caused to be incident upon the laser medium 2, and is absorbed by the medium. The beam that is transmitted through the medium without being absorbed, is reflected by the reflecting mirror 5, and is made incident upon the laser medium 2 once again. Energy of the absorbed light beam is made into an oscillating state by an optical resonator constituted of the partially reflecting film 4 and the totally reflecting film 3, and a part of the energy is emitted toward the outside as laser light beams.
The absorption coefficient of the laser medium 2 for the light from the LD 1 has a large wavelength dependency, which is, for example, 0.75 mm.sup.-1 for a radiation with wavelength of 808.5 nm and 0.1 mm.sup.-1 for a radiation of 802 nm. Therefore, it has been necessary to control precisely the spectrum of the LD 1 in order to realize an effective absorption of light by the laser medium.
The prior art laser device is constructed as above, so that it it is difficult for the light from the laser diode to be absorbed completely by the laser medium, and hence the energy efficiency of the laser oscillation remains low.
Further, in the prior art laser device, heat radiation from the laser medium is insufficient so that beam quality is deteriorated as the output increases.
Moreover, about 20% of the incident light from the LD 1 is reflected from the surface of the laser medium 2 or the reflecting mirror 5, and reenters the LD1, causing a disturbance to the LD and causing the output and the wavelength of the LD to be varied. FIG. 2 is a wavelength distribution diagram, which shows the disturbance due to reflected light, in which the abscissa is the wavelength of the LD light, and the ordinate is the intensity of the LD light. In the figure, curve A shows the wavelength distribution of the RD 1 when there is no laser medium 2, wherein there is obtained a waveform with a steep peak in the vicinity of 808 nm. Curve B is the wavelength distribution when the laser medium 2 is placed. It can be seen that both of the wavelength and the output of the LD light are disturbed due to the influence of the reflected light from the laser medium 2.
As described above, in addition to the low energy efficiency of the laser oscillation, the prior art laser device has a problem in that the reflected light from the laser medium 2 disturbs the LD 1.